PWM techniques have recently started to become prevalent in hi-fi audio, to give “Class D” or “switching” output stages. PWM power amplifiers also find application in a range of applications including motor control.
The high efficiency offered by these techniques minimises driver power dissipated in the driver transistors, so small and simple cheap packages can be used even for 100 W power amplifiers, rather than expensive special low-thermal-resistance packages. Savings in system cost and a smaller physical size result from the reduced need for heat-sinking. There are similar savings in cost and size from the reduced current rating required from the system power supply.
The higher acceptable package thermal resistance also tenders it practicable to use standard high-pin-count packages, for output power up to say 10 W, allowing other digital or small-signal analogue functions to be implemented on the same integrated circuit, thus reducing overall system cost weight and physical volume and improving reliability.
One problem with using a simple output stage is that, for a given input PWM duty-cycle, the output amplitude is directly proportional to the power supply voltage. Thus any ripple on the power supply will inter-modulate with the applied audio signal. Similarly, any signal-dependent ripple on the power supply will generate harmonics. Also the output driver stage will not be an ideal switch: the output transistors and pre-drivers will have finite and variable switch-on and switch-off times. Also there may be periods when inductive fly-back causes current to flow in output clamp diodes with non-linear and variable characteristics.
U.S. Pat. No. 4,249,136 (Suzuki et al) discloses an arrangement in which feedback is applied around the PWM amplifier, to suppress these modulation effects. The input signal is a 2-level PWM signal, which is compared to an attenuated version of the PWM output signal produced by the power-switch. The resulting error signal is filtered by an integrating loop filter which has a high gain in the audio signal frequency range but rejects higher frequencies, and compared to a zero voltage reference. The output of the comparator is used to control the power switch, which comprises a pair of power transistors. In this way, the output amplitude is now substantially defined only by the amplitude (and duty cycle modulation) of the PWM waveform applied to Vin, and becomes largely independent of power supply voltage.
However, deeper evaluation and practical experience reveals that this circuit only operates satisfactorily over a limited range of supply voltage, which constrains the accuracy requirement for the system power supply. Moreover, especially for low-cost applications, the power supply is likely to have poor load regulation at d.c. and even poorer load regulation at high audio frequencies. As the supply voltage falls due to load-current demands, the feedback causes the output pulse-width to increase to compensate to maintain the output audio-frequency signal amplitude. If the modulation index is high, the peaks of the values of the widths of the pulses can increase to the extent that adjacent output pulses collide with each other, causing a reducing in pulse-repetition frequency (PRF). This is a problem because it introduces lower-frequency carrier components which will not be adequately attenuated by the output filter and will generate objectionable non-harmonic audible distortion.
Also Class D amplifiers can suffer from an effect called ‘supply pumping’, where the supply voltage can increase as well as decrease, due to current being fed back into the supply from the output inductor during decreasing signals, as the inductor returns energy to the supply. The supply voltage may also rise if current demand suddenly falls, due the output inductance or low load regulation bandwidth of the power supply.
As also discussed in U.S. Pat. No. 6,140,875, a PWM amplifier with feedback control can exhibit high-frequency oscillations under these conditions. These high-frequency oscillations will disrupt the feedback loop, giving possibly gross audible distortion. They may also impair the efficiency of the output stage, possibly causing overheating or even destruction of the output stage. The only solution proposed in U.S. '875 is to keep the input modulation depth low, and configure the amplifier to have additional gain to compensate, but this limits the performance of the amplifier under normal conditions.